Paranoia, self-pity and loneliness are key characteristics of political leaders. Ravings about plots and conspiracies sometimes show a mind on the edge; Eden, Nixon and Wilson show the pattern. But more often paranoia is not so much a delusion as a rational response, since colleagues are so often dodgy vassals ever ready to dislodge their patron.

Some leaders are born mad, and where power is hereditary their societies have to tolerate them. Carlos the Bewitched was allowed to sit on the Spanish throne as Charles II from 1665 to 1700 because his country had given up on the idea of success. George III, ill rather than mad, showed a constitutional monarchy can function quite happily as long as the knowledge of a dynastic incapacity is confined to the inner circle. The specialised loopiness of democratic leaders, however, can't be concealed.

The neurosis may be private, but the consequences of choices disconnected from reality are there for all to see. This peculiarity is not demonstrated only in Thatcherish tantrums. Boring leaders can be mad too: Clement Attlee, given a huge mandate for social change in 1945, chose a cabinet of elderly dullards who contained the fizz in the bottle, failed to abolish public schools and ensured that Labour had lost the plot by 1947. Its only interesting member was Aneurin Bevan; unfortunately, he couldn't add up. Hence NHS deficits right from the start. How crazy was that?

The self-pity of leadership develops from day one in office. Tony Blair has told us that we would be awed by the scars on his back, and he has flayed himself through failed attempts at "public-sector reform". His martyrology focuses on himself because he belongs to a special class of leaders whose vision defines their distance from their parties. Pointless premierships - Callaghan's and Major's, for example - do the self-pitying act as well. But it's the self-pity of victimhood, telling us that outside forces, unions and party rebels, spoiled the show. Leaders guided by their star are different, and we are meant to envisage them voyaging through strange seas of politics but not, entirely, alone.

Leaders get detached from parties for many reasons. Boredom is one; Ramsay MacDonald found the Marchioness of Londonderry more exciting than the parliamentary Labour party. Blair prefers millionaires to marchionesses as dining companions, which counts for some as progress. Ted Heath opted for yachts, Lloyd George went golfing, and Gladstone built a library on his estate. Thatcher relaxed with her power-dresser ("Crawfie"), who kept the bows in place. Wreckers, all of them, for reasons both good and bad.

It is a party's function to be sterile and lag behind the social changes it tries to represent. It does not exist to promote thought and curiosity but to provide the policy options and institutional ladders that make possible the career structures of professional politicians. Britain is mostly relaxed about this state of affairs as long as it promotes the famous British stability. But the necessary introspection of a party can destroy it.

Labour, post-MacDonald, was an example of a self-pitying party obsessed by its betrayal. The careers of Asquithian Liberals and 70s Tories were overwhelmed by events they failed to understand: working-class politics in one case, capitalist crisis in the other. The leaders who wreck their own parties elevate, instead, a high view of executive power. Lloyd George, Gladstone and Thatcher were all markedly intolerant of democratic messiness and incompetence - like Blair today. Lloyd George's admiration of practical-minded businessmen whom he brought into government anticipates Blairite practice - right down to the buying of honours by the plutocracy.

The leader as wrecker sees, correctly, that in the circumstances of a party political breakdown, the lonely leader is an appealing figure. But the loner, in order to be also a successful leader, needs to build a new coalition of interests. This can be done through the old party's decayed structures. Gladstone's Liberals claimed a Whig legacy just as Thatcher's Conservatives claimed a Tory one. The striking fact about Blair is that, having run so far ahead of his own party, he is clueless about whom he represents. Gladstone had his base in the nonconformist vote. Thatcher had hers in a new class of homeowners. But there is no similar electoral group that considers its self-interest to be bound up with Blair's. All that is left is the fading sheen of that which once was new.